In aircraft, ovens can employ resistance heating and recirculated air for heating meals and/or drinks. As an alternative to resistance heating ovens, induction ovens can be used on aircraft. Compared with resistance heating ovens, induction ovens tend to be more efficient, thus consuming less electricity, can be made in a smaller design and with lighter weight, can heat the meals in a relatively shorter time, and tend to remain cooler on exterior surfaces.
For induction ovens in aircraft, there are additional design constraints that differ from conventional requirements for induction ovens that are not used on aircraft. For example, aircraft induction ovens typically weigh as little as possible; operate on the aircraft alternating voltage power supply system, that is, a three-phase alternating current typically having 115Veff from phase to neutral conductor, currently having frequencies of 400 Hz, but will in the future have frequencies that vary from 360 to 800 Hz; and produce only a small number of harmonic frequencies (harmonic oscillations).
The design constraint relating to harmonic frequencies is described in detail in J. Sun, “Analysis and Design of Single-Phase PFC Converters for Airborne Systems”, Industrial Electronics Society, 2003. IECON ′03, The 29th Annual Conference of the IEEE 2-6 Nov. 2003, P. 1101-1108. In order to comply with the design constraints on aircraft, Sun suggests use of active converters, so-called PFC (power-factor correction) converters, which shape the characteristic of the input current. A PFC converter includes an inverter that is controlled by a PFC controller. Inverters that are arranged downstream of a power supply rectifier are controlled by the PFC controller in such a manner that as few harmonic frequencies as possible are produced, that is, a current that is sinusoidal to the greatest possible extent is drawn from the power supply system. The PFC controllers that are available on the market are optimized for land-based electrical power supply systems with 50 and 60 Hz and can have defects at 400 Hz and in particular at up to 800 Hz.
The PFC controller described by Sun uses a measurement of the power supply input voltage and the power supply input current and controls the inverter in such a manner that the power supply input current follows the shape of the power supply input voltage as closely as possible. To this end, it is advantageous to have a correct measurement of the input voltage and the input current. As the power supply frequency increases, however, some problems occur in taking this measurement. In particular, occurrences of distortion in the current flow during zero crossing of the input voltage (zero-crossing distortion) can occur. Sun concentrates on the measurement errors that can occur during the measurement of the current. These errors become worse as frequencies increase and the increased requirements for aircraft power supply systems lead to additional measures that are taken in order to keep the harmonic frequencies that are produced under the required limits. Sun suggests phase correction, a so-called “adaptive current loop gain” or a “feedforward control.”
To prevent “zero-crossing distortion,” it is suggested in Application Note AN1214 “Design Tips for L6561 Power Factor Corrector in Wide Range” ST, PP. 1-6, December 2000 (Note AN1214) to use a transistor having a low Drain-Source Capacitance Coss in a boost converter that at the same time has a high turn-on resistance Rdson and a low breakdown voltage. Note AN1214 suggests using a transistor STP6NB50 from SGS-Thomson. Furthermore, Note AN1214 suggests to lower the switching frequency of the transistor to prevent zero-crossing distortion. However, this can lead to higher levels of inductance for the transformer of the converter and consequently to a higher weight.
The self-capacitance of the inductor of the boost converter can be reduced by using additional isolation strips or chambers in the inductor. The inductor of the boost converter corresponds to the transformer in the flyback converter.
The capacitance of the capacitor in the primary circuit can be reduced to reduce the distortion of the voltage shape. However, other occurrences of interference can couple into the device or decouple from the on-board power supply system when the capacitance is reduced too much.